Oxidative processes are a major contributing factor in senile cataracts. We have demonstrated that metal catalyzed oxidation of the crystallins induces protein modifications that mimic those seen in aging, senile cataracts and brunescent lenses. The importance of the role of metal in oxidative processes related to cataract is further supported by studies in other labs. The lens contains high levels of thiols such as, glutathione that can participate in these metal-catalyzed oxidation reactions. Our lab has continued studies on the mechanisms that protect the lens against deleterious oxidation reactions. A protein that protects enzymes specifically against inactivation by thiol-dependent metal-catalyzed reactions has been reported in yeast and rat tissues. (This activity is not related to catalase, glutathione peroxidase, or superoxide dismutase). Our lab demonstrated the presence of a similar activity in lenses of bovine, guinea pig, human, pig, monkey, and rat. The antioxidant activity consistently copurifies with a subpopulation of glutathione S-tranferase mu. To further understand the role of glutathione S-transferase in protecting agains oxidative stress the bovine enzyme was cloned. Three independent clones were sequenced yielding the complete sequence of the proein. Two of the clones were expressed in E.coli. The recombinant glutathione S-transferases are both active transferases but also provide protection against thiol dependent metal catalyzed oxidation of enzymes but not against ascorbate dependent metal catalyzed reactions.